A wire cut electric discharge machine has been extensively employed in the art in which a wire electrode is allowed to penetrate a workpiece to be machined such as a hard metal workpiece with a small gap therebetween, and while the wire electrode and the workpiece are being moved relative to each other along a complicated machining locus, a machining voltage is applied across the wire electrode and the workpiece to cause electric discharge therebetween, so that the workpiece is accurately machined with the discharge energy.
With the wire cut electric discharge machine, it is necessary to penetrate the workpiece with the wire electrode, to machine it.
A wire electrode is, in general, a thin metal wire, and therefore it is rather difficult to manually penetrate the workpiece with the wire electrode. The wire electrode is often broken by the occurrence of abnormal conditions during machining. Whenever the wire electrode is broken, the operator is forced to perform the difficult operation of inserting a wire electrode into a workpiece.
For instance in a progressive cutting operation, a workpiece having a plurality of cutting parts which are separate from one another, after a part of the workpiece has been cut, the wire electrode must be removed therefrom and inserted into workpiece at the next cutting operation start point. This difficulty obstructs automation of a wire cut electric discharge machine, and greatly lower the machining efficiency thereof.
In order to overcome the drawback, a wire electrode feeding device has been proposed in the art in which a wire electrode is driven by means of rollers or belts so as to be automatically inserted into a hole formed in the workpiece.
The conventional wire electrode feeding device is as shown in FIG. 12. In FIG. 12, reference numeral 1 designates a wire electrode; 2, a pipe guide; 3 and 4, pairs of wire driving rollers; 5a and 5b, current supplying dies; 6, a workpiece; and 7 and 8, electric motors for driving the pairs of rollers 3 and 4, respectively.
The wire electrode 1 led to the rollers 3 is driven by the latter so that it is inserted into the pipe guide 2. The wire guide 1 thus inserted is introduced to the lower rollers 4 through the workpiece 6 and the current supplying dies 5a and 5b.
When the wire electrode 1 is broken during machining, the breakage occurs at its part between the two pairs of rollers 3 and 4. In this case, the wire electrode is driven by the pair of rollers 3 again so that it is inserted into the workpiece 6.
The conventional wire electrode supplying device in a wire cut electric discharge machine is constructed as described above. On the other hand, there has been a significant tendency to use thinner wire electrodes to accomplish a wire cut electric discharge machining operation with higher accuracy. A thin wire electrode is low in mechanical strength, and accordingly if it is held strongly with the pairs of rollers, then it may be deformed, which adversely affects the machining accuracy. In order to drive the thin wire electrode satisfactorily, the pair of rollers must be high in mechanical accuracy. In order to meet the requirement, the pairs of roller must be increased in size, and accordingly they must be positioned remoter from the workpiece, and the pipe guide disposed between the pair of rollers and the workpiece must be also lengthened as much. In inserting the flexible wire electrode into the pipe guide thus lengthened, a frictional force is produced between the wire electrode and the guide pipe; that is, it is rather difficult to smoothly insert the wire electrode into the pipe guide.